An MRAM is a nonvolatile random access memory that utilizes a magnetoresistance element such as a magnetic tunnel junction (Magnetic Tunnel Junction; MTJ) element as a memory element. Since the MRAM is capable of achieving high-speed operation and can have infinite rewrite tolerance, research and development of the MRAM have been actively carried out to achieve commercialization in recent years. To further improve the versatility of the MRAM hereafter requires reduction in a write current and increase in a read-out signal. The reason is that the reduction in the write current not only lowers operation power consumption but also reduces costs due to reduction in a chip area, and the increase in the read-out signal shortens a read time which enables a higher-speed operation.
As a write method for reducing the write current, the following spin transfer magnetization switching method has been proposed. Let us consider, for example, a memory element in which a first magnetic layer having reversible magnetization, a nonmagnetic layer and a second magnetic layer whose magnetization direction is fixed are stacked. According to the spin transfer magnetization switching method, a write current is supplied between the second magnetic layer and the first magnetic layer, and interaction between spin-polarized conduction electrons of the write current and local electrons in the first magnetic layer causes switching of the magnetization of the first magnetic layer. To directly supply the current in the memory element at the time of data writing is one of major differences as compared with a typical write method; a magnetic field application method (a method that supplies a write current to an interconnection arranged near the memory element and applies a resultant magnetic field to switch the magnetization of the first magnetic layer). Moreover, in the case of the spin transfer magnetization switching method, the magnetization switching (writing) is caused when current density excesses a certain threshold value. Since the current density is increased as a cell size is reduced, the write current can be made smaller with miniaturization. That is, scaling property of the write current is improved. Japanese Patent Publication JP-2007-142364 (hereinafter referred to as “Patent Document No. 1”) discloses material characteristics which can make the threshold current density for the magnetization switching equal to or less than a desired value. According to it, it is possible to reduce the threshold current density by using a perpendicular magnetization film as a magnetic layer and adjusting magnetic anisotropy energy density and saturation magnetization as appropriate.
Whereas, to increase a magnetoresistance ratio (MR ratio) of the magnetoresistance element is most effective for increasing the read-out signal. Development of an MTJ element that exhibits a high magnetoresistance ratio has been actively performed in recent years. Hayakawa et al., “Effect of high annealing temperature on giant tunnel magnetoresistance ratio of CoFeB/MgO/CoFeB magnetic tunnel junctions”, APPLIED PHYSICS LETTERS, Vol. 89, p. 232510, 2006 (hereinafter referred to as Non-patent Document No. 1) reports that a giant MR ratio (about 500% at room temperature) can be obtained in the Co—Fe—B/Mg—O/Co—Fe—B MTJ. The reasons that such a high MR ratio can be obtained in the Co—Fe—B/Mg—O/Co—Fe—B MTJ are considered to be as follows: (1) Co—Fe—B has high spin polarization, (2) (001)-oriented polycrystalline MgO that exhibits high spin filtering effect is formed by annealing Mg—O sandwiched between amorphous Co—Fe—B at high temperature.
As described above, in the case of the spin transfer magnetization switching method, a perpendicular magnetization film whose material characteristics are adjusted as appropriate can be used as a magnetic layer in order to reduce the write threshold current density and hence reduce the write current of the MRAM. Whereas, an MTJ consisting of a magnetic layer having high spin polarization and an insulating layer exhibiting high spin filtering effect can be used in order to increase the MR ratio and hence increase the read-out signal of the MRAM.
However, when the perpendicular magnetization film having appropriate material characteristics is used as the magnetic layer for reducing the write threshold current density, it is difficult to develop an MTJ exhibiting a high MR ratio. The above-mentioned Patent Document No. 1 describes that the saturation magnetization being smaller is a preferable material characteristic of the perpendicular magnetization film. Whereas, the spin polarization of a magnetic layer generally becomes higher as the saturation magnetization is larger and becomes lower as the saturation magnetization is smaller. Therefore, high spin polarization cannot be obtained by the material characteristic preferable for the perpendicular magnetization film, which makes it difficult to improve the MR ratio. That is to say, reduction in the write threshold current density and improvement of the MR ratio in the spin transfer magnetization switching conflict with each other.
Moreover, a Co—Fe—B thin film having high spin polarization is an in-plane magnetization film whose magnetic anisotropy is in a direction parallel to the film surface. When such an in-plane magnetization film is used as a magnetic layer, it is difficult to sufficiently reduce the write threshold current density in the spin transfer magnetization switching.